The present invention relates to a structure and method for changing or controlling the thermal emissivity of the surface of a radiating object in situ, and thus, for changing or controlling the radiative heat transfer between the object and its environment in situ. More particularly, changing or controlling the degree of blackbody behavior of the object is accomplished by changing or controlling certain physical characteristics of a structure defining a plurality of cavities on the surface of the object. As described herein, this cavity structure may be integral to the radiating object or added to the surface of the object to form a new radiating surface.
Heat transfer between an object and its environment is achieved by up to three main processes: conduction, convection, and radiation. While conduction occurs at solid/solid and solid/fluid interfaces, the principal means of transferring heat into or out of many systems is by a combination of convective media and radiation. Terrestrial system designs typically exploit both convective and radiative heat transfer, however, heat management in many space (i.e., extraterrestrial) systems relies essentially on radiation because of the lack of a convective medium.
Convective heat transfer is provided by the natural or forced flow of a fluid over the surface of an object and can be controlled by changing parameters such as the fluid medium and/or its physical properties, flow rate, and surface roughness. In contrast, radiative heat transfer depends on the degree of blackbody behavior exhibited by the surface and the fourth power of surface temperature. Thermal energy radiated by a surface is expressed by the Stefan-Boltzmann equation:
Qrad=A"sgr"xcex5(Tb4xe2x88x92Ta4)xe2x80x83xe2x80x83(1)
where
Qrad=thermal power radiated (W)
A=area of radiating surface (m2)
"sgr"=the Stefan-Boltzmann Constant (5.67xc3x9710xe2x88x928W/m2/K4)
xcex5=thermal emissivity factor of radiating surface
Tb=temperature of the radiating surface (K)
Ta=ambient temperature (K)
The thermal emissivity factor (xcex5) is the ratio of an object""s radiative emission efficiency to that of a perfect radiator, also called a blackbody. The thermal emissivity factor of most materials ranges between 0.05 and 0.95 and is relatively constant over a significant temperature range. Therefore, the radiative heat transfer capability of an object is typically a predetermined, monotonic function of its temperature raised to the fourth power.
The following example illustrates the expected impact of changing the thermal emissivity, or degree of blackbody behavior, of an object that is transferring heat by free convection and radiation. In this example, the reference object is a horizontal cylinder 1 m long with a 10 cm outer diameter, rejecting heat to a 300K environment through free convection and radiation. A simplified equation for the laminar flow convective heat transfer coefficient, h, for the object is:
h=1.32(xcex94T/Dc)0.25xe2x80x83xe2x80x83(2)
(Holman, J. P., Heat Transfer, Sixth Edition, McGraw-Hill) where
xcex94T=temperature difference between surface and ambient (K)
Dc=diameter of cylinder (m)
Heat transferred by convection (Qconv) is expressed by:
Qconv=hA(Tbxe2x88x92Ta)xe2x80x83xe2x80x83(3)
where A, Tb, and Ta are the same variables as in Equation 1.
FIG. 1 shows the amount of heat rejected from the reference object by convection and radiation using Equations 1 and 3, respectively, over a xcex94T range of 1-1000 K, which covers a principal range of engineering interest. This figure shows the convection term (Qconv) to be approximately an order of magnitude larger than radiation (Qrad) from a surface with xcex5=0.1 for xcex94T up to about 100 K. Beyond this temperature, the T4 dependence of radiation increases more rapidly, making the two modes of heat transfer approximately equal when xcex94T approaches 1000 K. In contrast, radiation from a surface exhibiting ideal blackbody behavior (i.e., xcex5=1.0) is always greater than convection and is at least an order of magnitude larger when xcex94T is near or above 1000 K. More importantly, FIG. 1 illustrates the potential impact on the heat transfer capability of the reference object as the thermal emissivity of its surface changes, by changing the thermal emissivity factor from xcex5=1.0 to xcex5=0.1, and vice versa.
Thus, the ability to change or control the degree of blackbody behavior of a radiating object, while it is in service (i.e., in situ), analogous to changing or controlling the convective term in a fluid system during operation by altering the flow rate of the fluid, would enable a remarkable improvement in the thermal design and control of many systems where radiative heat transfer is important. For example, the surface of an object or system with controllable thermal emissivity could be activated at some limiting temperature as a thermal safety valve. In this mode of operation, the surface would be triggered to switch to a higher thermal emissivity that, in turn, radiates more heat to prevent the temperature of the object or system increasing above safe limits. Similarly, switching thermal emissivity to a lower value could protect against a system operating at less than a desirable temperature limit.
In addition, changing the thermal emissivity of an object will effectively change its thermal, or infrared (IR), signature. This is especially important in detection, recognition, and camouflage applications. For example, the ability to change or control the thermal emissivity of an object provides an opportunity for an object to match its thermal emission characteristics with those of other objects or structures in its vicinity, thereby enabling an IR camouflage effect.
In current systems where radiative heat transfer is important, the surface material and/or surface preparation of a radiating object is carefully selected to obtain the desired fixed thermal emissivity and resulting radiative heat transfer characteristic. Typical surface preparations include a variety of coating, etching, and polishing techniques. Etching techniques are also being used to create fixed surface textures for spectroscopic applications. For example, Ion Optics Inc. (Waltham, Mass.) has developed tuned infrared sources using ion beam etching processes that create a random fixed surface texture consisting of sub-micron rods and cones (http://www.ion-optics.com). Such a surface texture has a high emissivity over a narrow band of wavelengths and low emissivity in other bands and is an attractive alternative to IR light-emitting diodes.
Applying the emerging field of solid state microelectromechanical technology, tunable IR filters for IR spectral analysis are also being developed. An example of such a device is reported by Ohnstein, T. R., et al (xe2x80x9cTunable IR Filters With Integral Electromagnetic Actuators,xe2x80x9d Solid State Sensor and Actuator Workshop Proceedings, 1996, pp 196-199, Hilton Head, S.C.). Such tunable IR filters comprise arrays of waveguides whose transmittance can be varied by changing the spacing between them using linear actuators. The wavelength cutoff range from 8 xcexcm to 32 xcexcm achieved by Ohnstein et al with this technology is typical of its narrowband selectivity. Such IR spectral analysis devices, like the devices developed by Ion Optics, Inc., are purposely designed with surface microstructures having dimensions comparable to specific wavelengths in the electromagnetic spectrum to be effective at wavelengths that are discrete or in narrow bandwidths. Consequently, these devices are ineffective for applications which require the changing or controlling of broader ranges of wavelengths important in radiative heat transfer.
Accordingly, there is a need for a capability to change or control broadband radiative heat transfer between an object and its environment while the object is in service.
The present invention provides a structure and method for changing or controlling the thermal emissivity of the surface of an object in situ, and thus, changing or controlling the radiative heat transfer between the object and its environment in situ. Changing or controlling the degree of blackbody behavior of the object is accomplished by changing or controlling certain physical characteristics of a cavity structure on the surface of the object. The cavity structure, defining a plurality of cavities, may be formed by selectively removing material(s) from the surface, selectively adding a material(s) to the surface, or adding an engineered article(s) to the surface to form a new radiative surface.
The physical characteristics of the cavity structure that are changed or controlled in accordance with the present invention include cavity area aspect ratio, cavity longitudinal axis orientation, and combinations thereof. Controlling the cavity area aspect ratio may be performed by controlling the size of the cavity surface area, the size of the cavity aperture area, or a combination thereof. As described herein, the cavity structure may contain a gas, liquid, or solid that further enhances radiative heat transfer control and/or improves other properties of the object, for example surface finish, while in service.
The subject matter of the present invention is particularly disclosed and distinctly claimed in the concluding portion of this specification. However, both the organization and method of operation, together with further advantages and objects thereof, may best be understood by reference to the following description and examples taken in connection with accompanying drawings wherein like reference characters refer to like elements.